These call numbers are useful for browsing for euphonium music:

M 110  euphonium alone

M 270-271  duets for euphonium and another instrument

M 289  trios which include euphonium

M 457  quartets which include euphonium

Quintets, sextets, octets, and nonets follow the same call number pattern (M500s, 600s, 700s, 800s, and 900s)

Euphonium Concertos

M 1205  full scores: euphonium with wind ensemble

M 1206  concertos arranged for euphonium and piano
 

*The Petrucci Music Library (IMSLP) has many titles available in the public domain to download for free for full use.